1. Field of the Invention
The present invention relates to a semiconductor light-emitting device with a transparent conductive film and, more particularly, to a high-brightness semiconductor light-emitting device using the transparent conductive film as a current spreading layer.
2. Description of the Related Art
A light-emitting diode (herein referred to as LED) serving as a semiconductor light-emitting device, which is a high-brightness blue, green, orange, yellow or red LED, can be manufactured since a GaN-based or AlGaInP-based high-quality crystal can be grown by a MOVPE method in these years.
However, in order to achieve a high brightness, the current spreading layer must be increased in thickness such that an applied current is uniformly supplied into a chip plane of an LED. For example, in an AlGaInP-based LED device, the current spreading layer which is conventionally made of GaP or AlGaAs needs to have a large thickness of about 5 to 10 μm. Such a large amount of materials for the current spreading layer and a long processing (growth) time for the layer lead to a high manufacturing cost of the LED device.
In order to overcome the above problems, a method of using an ITO (Indium Tin Oxide) or ZnO (Zinc oxide) serving as a film having sufficient translucency and electric characteristics which can achieve preferable current spreading characteristics as a current spreading layer is proposed (see Japanese Patent Application Laid-open No. 8-83927). A method of directly forming an ITO film on a p-type clad layer is also proposed (see U.S. Reissue Pat. No. 35665 and U.S. Pat. No. 6,057,562).
As described above, an ITO film can be used as a current spreading layer, a conventional method of increasing a semiconductor layer serving as a current spreading layer to about 5 to 10 μm is not necessary, and, accordingly an epitaxial layer is not necessary. For this reason, a high-brightness LED device and an epitaxial wafer for an LED device can be manufactured at low cost.
However, when an ITO film is used as a window layer, a contact resistance is generated between a semiconductor layer and the ITO film comprising a metal oxide. And, there is a problem that a forward operation voltage disadvantageously increases. More specifically, the ITO film serving as a transparent conductive film (transparent electrode) comprises an n-type semiconductor. However, the upper clad layer being in contact with the ITO film comprises a p-type semiconductor. Therefore, when a forward operation voltage is applied to an LED, a reverse bias state is set between the transparent conductive film (transparent electrode) and the p-type clad layer. As a result, a current does not flow without applying a large voltage to the LED.
As a countermeasure against the problem, the following method is known. That is, a contact layer which is a thin layer with a high carrier density is formed between the ITO film and the p-type clad layer, and is in contact with the ITO film. Resultantly, an LED can be driven at a low voltage by a tunnel junction (for example, U.S. Reissue Pat. No. 35665). However, since the p-type clad layer has a small thickness (i.e., when the p-type layer has only a thickness of 200 nm or more, it can function sufficiently as a clad layer), Zn serving as a dopant is likely to be diffused from the contact layer into an active layer.
The diffusion of the p-type dopant from the contact layer causes the following two problems. The first problem is a decrease in optical output of an LED device. When the dopant is diffused in a depth direction of the LED device and reached to an active layer of the LED device, the dopant becomes a defect in the active layer. The defect works as a nonradiative recombination component, and its optical output decreases with time. The second problem is an increase in drive voltage of the LED device. Since a substantial carrier density of the thin contact layer decreases due to the dopant diffusion, the tunnel junction cannot be easily achieved. For this reason, the drive voltage of the LED device disadvantageously increases.
As a method for solving the problems of the diffusion of a dopant, it is known to increase the distance between an active layer and a contact layer, i.e., to increase the thickness of a p-type clad layer, so as to suppress the diffusion of the dopant into the active layer. In this method, the same material as the p-type clad layer must be grown at a low growth rate, and the cost of the material must be increased.
On the other hand, U.S. Pat. No. 6,057,562 discloses a method that a buffer layer (to improve the current spreading) comprising, e.g., an AlGaAs layer, is formed between a p-type clad layer and a contact layer to enhance the current spreading to increase the optical output.
However, the inventors have found that, when conducting the method in U.S. Pat. No. 6,057,562 by using an AlGaAs layer etc. with a high Al ratio in mix crystal as a material comprising the buffer layer, dopant is easy to be diffused due to a heat during growth etc. and the degree of dopant diffusion becomes conspicuous. Therefore, in order to prevent the diffusion of the dopant from the contact layer into its active layer, the buffer layer must be increased in thickness.